Article 'a' with 'night' and 'day': It was opening night


The following sentences have been taken from a novel. Could yo please tell me where the article is before night and day? :shock: :shock: :shock:

1- It was opening night. (why not an opening night?)
2- It’s market day in Cortona. (why not a market day?)


They are the names of particular nights and days: Christmas Day, parents’ day (at school), etc. (Also: tomorrow night, yesterday).

However, they can also appear as the opening night, the market day– here they are not named days, but descriptive adjectives for the days.